Braking of a vehicle is normally required for two reasons—to decelerate the vehicle when the vehicle is in motion (generally referred to as service braking), or to ensure that the vehicle does not move when it is parked. Service braking is usually effected by the driver operating a foot pedal, with a separate lever, usually manually operable, being provided to actuate and hold the brake or brakes on whilst the vehicle is parked. In both cases, however, each vehicle brake is typically moved to the applied position by means of a fluid pressure operable brake actuator.
Two sorts of fluid pressure operable brake actuators are known—a lock actuator, and a spring brake actuator. In the both of these, service braking is achieved by the movement of a piston or diaphragm which divides a service brake housing into first and second chambers. The piston or diaphragm carries an actuating rod, which extends from the first chamber and through an aperture in the service brake housing, and which is mechanically connected to a brake. In order to apply the brake, a fluid pressure (typically pneumatic) braking operating signal is supplied to the second chamber, and this causes the piston or diaphragm to move so that the actuating rod is pushed out of the housing to an extended position. A return spring is provided in the first chamber to return the piston or diaphragm and actuating rod to the retracted position when the fluid pressure is exhausted from the second chamber.
In the case of a lock actuator, operation of the parking brake causes a locking device to operate to mechanically lock the actuating rod (and possibly also the piston/diaphragm) in the extended position. The brake will thus remain applied even if fluid pressure is exhausted from the second chamber, until the parking brake, and hence the lock is released. There are, however, problems with existing embodiments of lock actuator, in particular when used to apply a very hot brake, as may occur shortly after stopping a laden vehicle previously travelling at high speed. Specifically, when used to actuate a drum brake, it was found that, if the vehicle was left with the parking brake applied for some time, shrinkage of the drum upon cooling could cause an increase in the brake force to such a level that the brakes become difficult if not impossible to release and/or the drum distorted or cracked. Alternatively, when used to actuate a disc brake, shrinkage of the disc upon cooling could cause the braking force to reduce over time to such an extent that a vehicle parked on a hill could be permitted to roll down the hill.
In view of these significant disadvantages, it is more common to use a spring brake actuator, particularly in heavy goods vehicles. In a spring brake actuator, rather than the actuating rod being mechanically locked in position when the parking brake is actuated, it is urged into the extended position by means of a heavy duty spring which is provided in a spring brake housing.
The spring extends between the spring brake housing and a further piston or diaphragm which divides the spring brake housing into two chambers—hereinafter referred to as the third and fourth chambers. The spring is located in the third chamber and the fourth chamber lies between the third chamber and the second chamber in the service brake housing. The spring brake piston/diaphragm is provided with an actuating rod which extends from the fourth chamber into the second chamber, where, under the biasing force of the spring, it pushes the service brake piston/diaphragm into the extended (i.e. brake applied) position. In order to release the brake, pressurised fluid is supplied to the fourth chamber to move the spring brake piston/diaphragm against the biasing force of the spring, which allows the service brake piston/diaphragm to return to the retracted position.
Whilst this arrangement does not suffer from the drawbacks of a lock actuator described above, it has to be relatively large to accommodate the additional chambers and a spring large enough to provide the desired braking force. It is an object of the invention to provide an actuator which is relatively compact and which minimises or avoids the problems associated with conventional lock actuators.